1. Field of the Invention
The invention relates to capacitor-based digital-to-analog converters (DAC), and in particular to a DAC operable at a low supply voltage.
2. Description of the Related Art
Current hi-fi audio applications generally record audio data in a digital form such that playback requires digital-to-analog conversion. During playback, over-sampling DACs have become popular due to low cost, high performance and high yield. Direct-charge transfer is one of the most general methods of implementing over-sampling DACs because of insensitivity to clock jitter and low slew rate requirement.
FIG. 1 shows a conventional direct-charge transfer (DCT) DAC 10. DCT DAC 10 is a kind of switched-capacitor circuit. As shown in FIG. 17 several switches S0-S4 cooperate with capacitors C1 and C2, and an operational amplifier (OP), where the switches control interaction between all other elements therein. In order to integrate with other circuits in a chip, DCT DAC 10 is usually implemented by way of CMOS fabrication technology, where a PMOS transistor, a NMOS transistor and a pass gate (the combination of one PMOS and one NMOS) are common candidates for implementing a switch. MOS switches, however, experience difficulty conducting signals at levels near half of the supply voltage especially when the supply voltage is low. Therefore, it is not easy to design a DCT DAC with a low supply voltage.